Method for treating surface of semiconductor layer, semiconductor substrate, method for making epitaxial substrate

ABSTRACT

A surface treatment method for a semiconductor layer includes growing a first layer on a substrate in a growth reactor, the first layer consisting of one of gallium nitride, aluminum gallium nitride and indium aluminium nitride; growing a second layer of gallium nitride on a surface of the first layer, the gallium nitride of the second GaN layer having a composition ratio of gallium to nitrogen larger than 2; taking the substrate out of the growth reactor after growing the second layer; and removing the second layer after taking the substrate out of the growth reactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for treating the surface of asemiconductor layer, a semiconductor substrate, and a method for makingan epitaxial substrate, in particular to a method for treating a surfaceof a semiconductor layer including a nitride semiconductor layer, asemiconductor substrate including a nitride semiconductor layer, and amethod for making an epitaxial substrate including a nitridesemiconductor layer.

2. Related Background Art

A semiconductor device including a nitride semiconductor is applied to,for example, a power device which operates at a high frequency and ahigh power. A high electron mobility transistor (HEMT) is known as thesemiconductor device that includes the nitride semiconductor. The HEMTincludes an electron transit layer and an electron supply layer.

Japanese Patent Application Laid-open No. 2011-3677 discloses thatforeign particles adhere onto an epitaxial wafer.

SUMMARY OF THE INVENTION

In the growth of a nitride semiconductor layer including gallium (Ga),particles containing Ga, such as droplets, sometimes adhere onto thenitride semiconductor layer to form accreting objects. When the nitridesemiconductor layer is grown to form a semiconductor device, thefollowing defects may be caused by the droplets: an electricshort-circuit is caused in the semiconductor device; breaking of wiresis caused in the semiconductor device; and unevenness of an appliedresist in thickness is caused in a fabrication process for thesemiconductor device. These defects result in the reduction in yield ofthe semiconductor device.

Some aspects of the present invention have an object to provide asemiconductor substrate and a method for reducing particles, whichcomprises gallium, adhering onto a semiconductor layer, and a method formaking an epitaxial substrate having the reduced number of particlescomprising gallium thereon.

A surface treatment method for a semiconductor layer according to oneaspect of the present invention includes the steps of growing a firstlayer on a substrate in a growth reactor, the first layer consisting ofone of gallium nitride, aluminum gallium nitride and indium aluminiumnitride; growing a second layer of gallium nitride on a surface of thefirst layer, the gallium nitride of the second GaN layer having acomposition ratio of gallium to nitrogen larger than 2; taking thesubstrate out of the growth reactor after growing the second layer; andremoving the second layer after taking the substrate out of the growthreactor.

A semiconductor substrate according to another aspect of the presentinvention includes: a first layer consisting of one of gallium nitride,aluminum gallium nitride and indium aluminium nitride; and a secondlayer of gallium nitride having a composition ratio of gallium tonitrogen larger than 2, the second layer being provided on the firstlayer.

Still another aspect of the present invention relates to a method ofmaking an epitaxial substrate. The method comprises the steps of:growing a first layer on a substrate in a growth reactor, the firstlayer consisting of one of gallium nitride, aluminum gallium nitride andindium aluminium nitride; growing a second layer of gallium nitridecompound on the first layer in the growth reactor, the first layer beingin contact with the second layer, and the gallium nitride compoundhaving a composition ratio of gallium to nitrogen larger than 2; takingthe substrate out of the growth reactor after growing the second layer;and etching the second layer to expose the first layer after taking thesubstrate from the growth reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and other objects, features, and advantages of the presentinvention will be more easily clarified from the following detaileddescription of a preferred embodiment of the present invention, whichproceeds with reference to the accompanying drawings.

FIG. 1 is a view showing a method of growing a semiconductor layer on asubstrate;

FIG. 2 is a view showing the method of growing the semiconductor layeron the substrate;

FIG. 3 is a view showing the method of growing the semiconductor layeron the substrate;

FIG. 4 is a sectional view showing a semiconductor substrate on whichdroplets are located;

FIG. 5 is a schematic view of an optical microscope image of thedroplet;

FIG. 6 is a view showing an EDX analysis result of the droplets;

FIG. 7 is a view showing the relationship between the size of thedroplets and the treatment time;

FIG. 8 is a view showing the relationship between a film thickness of aGaN layer and the treatment time;

FIG. 9 is a schematic view showing an optical microscope image of atrace of a droplet;

FIG. 10A is a sectional view showing a method of fabricating asemiconductor device according to Example 1;

FIG. 10B is a sectional view showing the method of fabricating thesemiconductor device according to Example 1;

FIG. 10C is a sectional view showing the method of fabricating thesemiconductor device according to Example 1;

FIG. 10D is a sectional view showing the method of fabricating thesemiconductor device according to Example 1;

FIG. 11A is a sectional view showing the method of fabricating thesemiconductor device according to Example 1;

FIG. 11B is a sectional view showing the method fabricating thesemiconductor device according to Example 1;

FIG. 11C is a sectional view showing the method of fabricating thesemiconductor device according to Example 1;

FIG. 12 is a schematic view of an optical microscope image observedafter the step shown in FIG. 11A;

FIG. 13 is a schematic diagram of an optical microscope image observedafter the step shown in FIG. 11B; and

FIG. 14 is an example showing an XPS measurement result on the surfaceof a GaN layer on which sulfur remains.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, embodiments according to some aspect of the resent invention willbe explained below.

One aspect of the present embodiments is a surface treatment method fora semiconductor layer, which comprises the steps of: growing a firstlayer on a substrate in a growth reactor, the first layer consisting ofone of gallium nitride, aluminum gallium nitride and indium aluminiumnitride; growing a second layer of gallium nitride on a surface of thefirst layer, the gallium nitride of the second GaN layer having acomposition ratio of gallium to nitrogen larger than 2; taking thesubstrate out of the growth reactor after growing the second layer; andremoving the second layer after taking the substrate out of the growthreactor. In the method according to the one aspect, growing a secondlayer on a surface of the first layer includes a step of forming thesecond layer on the first layer using an MOCVD reactor at a substratetemperature equal to or lower than 800 degrees Celsius. In the methodaccording to the one aspect, removing the second layer includes removingthe second layer using mixed liquid including sulfuric acid and hydrogenperoxide. In the method according to the one aspect, the growth reactorincludes an introducing port for introducing gas above the substrate. Inthe method according to the one aspect, an accreting object are formedon the second layer after growing the second layer, and the accretingobject is removed in the step of removing the second layer. In themethod according to the one aspect, the first layer and the second layerare continuously grown in the growth reactor.

Another aspect of the present embodiments relates to a semiconductorsubstrate comprising: a first layer consisting of one of galliumnitride, aluminum gallium nitride and indium aluminium nitride; and asecond layer of gallium nitride having a composition ratio of gallium tonitrogen larger than 2, the second layer being provided on the firstlayer. In the semiconductor substrate according to the another aspect,the semiconductor substrate further comprises particles containinggallium, the particles being provided on the second layer. In thesemiconductor substrate according to the another aspect, a filmthickness of the second layer is equal to or larger than 100 nm. In thesemiconductor substrate according to the another aspect, the secondlayer is formed to be in contact with the first layer.

Still another aspect of the present embodiments is a method for makingan epitaxial substrate, which comprises the steps of growing a firstlayer on a substrate in a growth reactor, the first layer consisting ofone of gallium nitride, aluminum gallium nitride and indium aluminiumnitride; growing a second layer of gallium nitride compound on the firstlayer in the growth reactor, the first layer being in contact with thesecond layer, and the gallium nitride compound having a compositionratio of gallium to nitrogen larger than 2; taking the substrate out ofthe growth reactor after growing the second layer; and etching thesecond layer to expose the first layer after taking the substrate fromthe growth reactor. In the method according to the above aspect, growinga second layer on a surface of the first layer includes a step offorming the second layer on the first layer using an MOCVD reactor at asubstrate temperature equal to or lower than 800 degrees Celsius. In themethod according to the above aspect, removing the second layer includesremoving the second layer using mixed liquid including sulfuric acid andhydrogen peroxide. In the method according to the above aspect, thegrowth reactor includes an introducing port for introducing gas abovethe substrate. In the method according to the above aspect, an accretingobject are formed on the second layer after growing the second layer,and the accreting object is etched in the step of etching the secondlayer.

One aspect of the present embodiments is a surface treatment method fora semiconductor layer including: forming a first GaN layer of galliumnitride on a substrate in a growth reactor; forming a second GaN layerof gallium nitride on the surface of the first GaN layer, the galliumnitride of the second GaN layer having a composition ratio of gallium tonitrogen larger than 2; taking the substrate out of the growth reactorafter forming the second GaN layer; and removing the second GaN layerafter taking the substrate out of the growth reactor. The second GaNlayer of gallium nitride having the composition ratio of gallium tonitrogen larger than 2 is grown on the first GaN layer to form anepitaxial substrate, and the epitaxial substrate is taken out of thegrowth reactor and the second GaN layer is removed from the epitaxialsubstrate. The removal of the second GaN layer allows accreting objectsor particles containing gallium from disappearing along with the secondGaN layer.

It is preferable that forming a second GaN layer of gallium nitrideinclude forming the second GaN layer on the first GaN layer at asubstrate temperature equal to or lower than 800 degrees Celsius in anMOCVD reactor. This temperature range allows the gallium nitride of thesecond GaN layer to have the composition ratio of gallium to nitrogenlarger than 2.

It is preferable that mixed liquid including sulfuric acid and hydrogenperoxide be used in the step of removing the second GaN layer.Consequently, the second GaN layer together with fallen objects orgallium-containing particles thereon can be removed.

The growth reactor may include an introducing port for introducing gaslocated above the substrate. This method allows for the reduction in thenumber of the fallen objects even in the above growth reactor, which maycreate the fallen objects.

It is preferable that, after forming the second GaN layer, the fallenobjects be formed on the second GaN layer and that the fallen objects beremoved in the step of removing the second GaN layer. This method allowsfor the reduction in the number of fallen objects remaining thereon.

It is preferable that, in the growth reactor, the first GaN layer andthe second GaN layer be continuously formed.

Another aspect of the present embodiments relates to a semiconductorsubstrate including a first GaN layer of gallium nitride; and a secondGaN layer formed on the first GaN layer, gallium nitride of the firstGaN layer having a composition ratio of gallium to nitrogen larger than2. According to the present invention, removing the second GaN layer canremove particles containing gallium thereon.

It is preferable that the semiconductor device include particlescontaining gallium fallen on the second GaN layer.

It is preferable that a film thickness of the second GaN layer be equalto or larger than 100 nm.

Details explanation of embodiments of the present invention will be madebelow. Specific examples of a semiconductor device, the method forfabricating the semiconductor device, and a semiconductor substrateaccording to the embodiments of the present invention are explainedbelow with reference to the drawings. Note that the present invention isnot limited to the specific examples. It is intended that all changeswithin meanings and scopes indicated by the claims and their equivalentsare included.

First, explanation on droplets (fallen objects: metal objects that peelfrom a gas introducing section (e.g., a showerhead) or the sidewall(e.g., a susceptor sidewall) of a growth reactor to adhere to asubstrate surface) are made. FIGS. 1 to 3 are schematic diagrams eachshowing a method of forming a semiconductor layer on a substrate. Asshown in FIG. 1, a metal organic chemical vapor deposition (MOCVD)apparatus can be used as a growth reactor 100, which includes a chamber50, an introducing section 52, a susceptor 54, a heater 56, and anexhaust section 58. The chamber 50 can hold a wafer 60 in an atmosphereof a raw material gas 62. The introducing section 52 includes, forexample, a showerhead, and can introduce the material gas 62 into thechamber 50. The introducing section 52 is provided to supply thematerial gas 62 to the surface of the wafer 60. The material gas 62 fromthe introducing section 52 is uniformly supplied to the surface of thewafer 60 to allow the entire surface of a semiconductor layer grown onthe wafer 60 to have a good uniformity in thickness. The susceptor 54holds one or plural wafers 60. The heater 56 heats the wafers 60 throughthe susceptor 54. The exhaust section 58 exhausts the material gas 62 inthe chamber 50 and operates to keep the pressure in the chamber 50constant.

In this embodiment, the growth reactor 100 has, for example, a face-uptype structure. The material gas 62 flows downward from above the wafer60 to the surface of the wafer 60 to reach the wafer 60 in a down-flowmanner. The material gas 62 around the wafer 60 is decomposed by thermalenergy that the heater 56 generates, so that the decomposed materialdeposits on the wafer 60 to grow a semiconductor layer on the wafer 60.

As shown in FIG. 2, a semiconductor layer 64 is formed on the wafer 60.The semiconductor layer 64 is made of single crystal and, in thisexample, is constituted by a nitride semiconductor layer. The susceptor54 is heated by the heater 56 to decompose the material gas 62 near thesusceptor 54. An amorphous or polycrystalline semiconductor layer 66 iscreated and adheres onto the susceptor 54. Further, the introducingsection 52 is also heated by heat radiation from the susceptor 54, andthe material gas 62 is also decomposed near the introducing section 52,so that the amorphous or polycrystalline semiconductor layer 66 alsodeposits on the lower surface of the introducing section 52. When thesemiconductor layer 64 is grown to form a semiconductor layer includingGaN, the semiconductor layer 66 is also grown such that a Ga compositionratio of the semiconductor layer 66 is higher than a Ga compositionratio of the semiconductor layer 64.

When a flow of gas changes in the chamber 50, as shown in FIG. 3, one ormore parts of the semiconductor layer 66 adhering to the underside ofthe introducing section 52 are detached therefrom and drop onto thesemiconductor layer 64. Consequently, droplets 68 are formed on thesemiconductor layer 64. The change in the flow of the gas is caused inthe chamber 50 for example, by the switching of the material gas 62, itsstop, its introduction, or the like.

In a growth reactor of a face-up type, as shown in FIGS. 1 to 3, inwhich the introducing section 52 is located above the wafer 60, piecesof the semiconductor layer 66 that adheres to the introducing section 52may fall with the force of gravity to reach the semiconductor layer 64.In a growth reactor in which the introducing section 52 is not locatedabove the wafer 60, the semiconductor layer 66 also deposits on theceiling of the chamber 50, which is located above the wafer 60. In theabove configuration, the droplets 68 are also farmed on thesemiconductor layer 64. Further, in a growth reactor different from thatof the face-up type, the droplets 68 from the semiconductor layer 66also sometimes adhere onto the semiconductor layer 64.

FIG. 4 is a schematic cross sectional view showing a semiconductorsubstrate to which the droplets have adhered. As shown in FIG. 4, thesemiconductor layer 64 is formed on the wafer 60, and the droplets 68adhere onto the semiconductor layer 64. Some of the droplets 68 haveheight H equal to or larger than, for example, 100 μm. Therefore, thedroplets located across conductive layers of the semiconductor deviceare likely to form an undesired electric short-circuit in thesemiconductor device, thereby resulting in a cause of defectiveproducts. The droplets are likely to cause breaking of wire in thesemiconductor devices. Further, a photo resist is applied to the wafer60 with droplets in a manufacturing process for the semiconductordevice, and the droplets cause unevenness in thickness of the appliedresist. As seen from the above examples, the droplets reduce the yieldof the semiconductor device.

FIG. 5 is a schematic view showing an image of the droplet obtained byan optical microscope. Referring to FIG. 5, the droplet 68 is drawn tohave a substantially spherical shape on the semiconductor layer 64. Thedroplets 68 on the surface of the wafer 60 or semiconductor piecespeeling from the semiconductor layer 66 are not spherical in most cases.The semiconductor pieces adhering onto the semiconductor layer 64 are,however, deformed by hest from the heater 56. The droplets 68 becomespherical to have the largest surface area according to surface tension.However, depending upon a substrate temperature after the adhesion orpieces onto the semiconductor layer 64, the droplets 68 may not have aspherical shape.

One of measures for preventing the droplets from being formed is methodof frequently cleaning the growth reactor, but this method decreases athroughput in the growth reactor. Another measure is a method ofremoving the droplets on the semiconductor layer 64 by blowing gas, butthe droplets are not efficiently removed only by the blowing of the gas.This is because the droplets are bound with the semiconductor layer 64through thermal reaction or intermolecular force.

Still another measure is a method of removing the droplets 68 by dryetching. This method may physically or chemically damage thesemiconductor layer 64. Since the surface of the semiconductor layer 64is exposed to plasma from the dry etching, it is likely to form analtered layer in the semiconductor layer 64.

A remaining measure is a method of removing the droplets 68 by wetetching, and the inventor examines the removal of the droplets 68 by wetetching.

A GaN electron transit layer, an AlGaN electron supply layer, and a GaNcap layer were grown on a substrate to form the monocrystallinesemiconductor layer 64. In this growth, droplets were formed to adhereonto the semiconductor layer 64, and were analyzed using an energydispersive X-ray spectroscopy (EDX) method.

FIG. 6 is a view showing an EDX analysis result of the droplets. Theabscissa indicates energy and the ordinate indicates the number ofcounts, and the solid line in FIG. 6 represents an analysis result ofthe droplets and the broken line represents an analysis result of themonocrystalline GaN layer. In the EDX analysis, there is littledifference in the peak intensity of gallium (Ga) around 1.1 keV betweenthe droplets and the GaN layer. On the other hand, the peak intensitynitrogen (N) around 0.4 keV is small in a signal from the droplets ascompared with the GaN layer. The comparison between signal intensitiesfrom gallium near 1.1 keV and nitrogen near 0.4 keV shows that acomposition ratio ([Ga]/[N]) of Ga and N in the droplets 68 is higherthan the composition ratio in the monocrystalline GaN layer. Thedroplets 68 are particles with, for example, the amount of Ga beingrich. For example, when the droplets 68 are a gallium nitride compound,a composition ratio of gallium to nitrogen is larger than two (i.e., thedroplets 68 include a compound Ga_(1+X)N, where X>2). The compositionratio of gallium to nitrogen is sometimes larger than three, and issometimes larger than four.

Ga can be wet-etched with mix liquid of sulfuric acid and hydrogenperoxide (sulfuric acid/hydrogen peroxide mixture), and the droplets onthe GaN monocrystalline layer were immersed in this sulfuricacid/hydrogen peroxide mixture. This sulfuric acid/hydrogen peroxidemixture contains sulfuric acid (96 volume %) and hydrogen peroxide (30volume %) at the ratio of sulfuric acid)(hydrogen peroxide)=5/1.

FIG. 7 is a view showing a relation between the size of the droplets onthe GaN monocrystalline layer and a treatment time during which thedroplets were etched using the sulfuric acid/hydrogen peroxide mixture.FIG. 8 is a view showing a relation between the treatment time and thefilm thickness of the GaN layer. The treatment time is defined as theperiod of time during which the droplets were immersed in the sulfuricacid/hydrogen peroxide mixture. The droplet size is specified as thesize of the droplets measured with a microscope. The film thickness ofthe GaN layer is specified as the film thickness of the GaNmonocrystalline layer measured using an X-ray diffraction (XRD) method.

As shown in FIG. 7, the size of droplets as etched decreases as thetreatment time increases. After the treatment time of 10 minutes, thedroplet size becomes zero, which indicates that the droplets have beenremoved.

As shown in FIG. 8, the film thickness of the GaN layer hardly changeseven after the treatment time of 20 minutes. This shows that thesulfuric acid/hydrogen peroxide mixture etches the droplets and does notetch the GaN monocrystalline layer. As shown in FIGS. 7 and 8, it ispossible to remove the droplets with the sulfuric acid/hydrogen peroxidemixture without etching the GaN monocrystalline layer.

The surface of the GaN layer from which the droplets were removed wasobserved using an optical microscope. A result is as explained below.FIG. 9 is a schematic view of an optical microscope imago showing atrace of a droplet. As shown in FIG. 9, the observation reveals that atrace 70 is left on the surface of the semiconductor layer 64 and thatthe trace 70 has a concave shape with depth of, for example, about 100nm. The trace 70 may cause a deficiency in the semiconductor device.

As seen from the above explanation, it is not easy to reduce the numberof droplets. Examples of the present invention will be explained belowwith reference to the drawings.

EXAMPLE 1

FIGS. 10A to 11C are schematic cross sectional views showing the primarysteps in a method of fabricating a semiconductor device according to anexample 1. As shown in FIG. 10A, a substrate 10 is prepared, and thesubstrate 10 can be, for example, a silicon carbide (SiC) substrate. Asthe substrate 10, for example, a silicon (Si) substrate, a sapphiresubstrate, a GaN substrate, or a gallium oxide (Ga₂O₃) substrate canalso be used.

As shown in FIG. 10B, a buffer layer 12, an electron transit layer 14,and an electrode supply layer 16 are grown on the substrate 10 using thegrowth reactor 100 shown in FIG. 1, thereby forming a nitridesemiconductor layer 18. In this embodiment, the buffer layer 12 can be,for example, an aluminum nitride (AlN) layer, The electron transit layer14 can be, for example, a GaN layer. The electron supply layer 16 canbe, for example, an AlGaN layer.

As shown in FIG. 10C, a GaN can layer 20 is grown on the nitridesemiconductor layer 18. The GaN cap layer 20 is a monocrystalline GaNlayer. Examples of growth conditions for the GaN cap layer 20 are aslisted below.

-   -   Substrate temperature: 1060 degrees Celsius.    -   Raw material gas: trimethylgallium (TMG), ammonium (NH₃)    -   TMG flow rate: 42 sccm (7×10⁻⁷ m/s)    -   NH₃ flow rate: 20000 sccm (3.3×10⁻⁴ m/s)    -   Carrier gas; hydrogen (H₂)    -   Pressure: 100 Torr (13.3 kPa)    -   Film thickness: 1 nm to 10 nm        The GaN cap layer 20 is grown to suppress oxidation of the        electron supply layer 16. It is preferable that, in order to        suppress the oxidation, the film thickness of the GaN cap layer        20 be equal to or larger than 1 nm. Triethylgallium (TEG) can be        also used as Ga raw material other than TMG.

As shown in FIG. 10D, a GaN compound is grown on the GaN cap layer 20 toform a low-crystallinity gallium nitride layer 22 with lowcrystallinity. A Ga composition ratio of the low-crystallinity galliumnitride layer 22 is higher than the Ga composition ratio of the GaN caplayer 20. In this embodiment, the atomic composition ratio of [Ga]/[N]in the monocrystalline GaN is substantially equal to one and the atomiccomposition ratio of Ga/N in the low-crystallinity gallium nitride layer22 is larger than one, so that the low-crystallinity gallium nitridelayer 22 is amorphous and/or polycrystalline. Examples of growthconditions for the low-crystallinity gallium nitride layer 22 are aslisted below.

-   -   Substrate temperature: lower than 900 degrees Celsius.    -   Film thickness: 100 nm or more and 2 μm or less.    -   Other conditions: same as the conditions for the GaN cap layer        20.

The decomposition temperature of NH₃ is about 900 degrees Celsius.Hence, the GaN cap layer 20 is grown at a substrate temperature equal toor higher than 1000 degrees Celsius, so that the GaN cap layer 20 is amonocrystalline layer. On the other hand, the low-crystallinity galliumnitride layer 22 is grown at a substrate temperature lower than 900degrees Celsius. Specifically, the substrate temperature is preferablyequal to or lower than 800 degrees Celsius, and more preferably equal toor lower than 750 degrees Celsius. The substrate temperature ispreferably equal to or higher than 600 degrees Celsius so that it ispossible to form a Ga-rich gallium nitride compound layer because thesupply of N is not markedly reduced.

The steps shown in FIGS. 10B to 10D are sequentially performed withoutstopping gas supply from the introducing section 52 to be a continuousgrowth process. For example, the GaN cap layer 20 and thelow-crystallinity gallium nitride layer 22 are continuously grown in thegrowth reactor. The final growth is not the step to grow the GaN caplayer 20 and is the step to grow the low-crystallinity gallium nitridelayer 22 of a gallium nitride compound. Consequently, the semiconductorlayer 66 is likely to be formed near the introducing section 52 as shownin FIG. 2, and the continuous growth steps can suppress the dropletsfrom dropping from the semiconductor layer 66 onto the nitridesemiconductor layer 18 or the GaN cap layer 20, as shown in FIG. 3,during the time after the growth of the nitride semiconductor layer 18or the GaN cap layer 20 and before the growth, of the gallium nitridecompound layer.

After the growth in the example is completed, as shown in FIG. 11A, thesubstrate 10 is extracted from the growth reactor 100. Prior to takingout the substrate 10 from the growth reactor, the gas introduced fromthe introducing section 52 is stopped, and the stopping of the gaschanges a flow of gas in the chamber 50. This gas stop may causedroplets 24 to drop onto the low-crystallinity gallium nitride layer 22to form a semiconductor substrate 102 (a substrate product or anepitaxial substrate). The semiconductor substrate 102 is stored withoutchange, and then the semiconductor substrate 102 is conveyed to afactory where a subsequent process is applied thereto. In this way, thesemiconductor substrate 102 is fabricated and the low-crystallinitygallium nitride layer 22 is formed on the GaN cap layer 20 thereof. Thissemiconductor substrate 102 allows for the protection (scratches andadhesion of accreting objects or the like can be prevented) of thesurface of the GaN cap layer 20.

As shown in FIG. 11B, the droplets 24 and the low-crystallinity galliumnitride layer 22 are removed using the sulfuric acid/hydrogen peroxidemixture. The sulfuric acid/hydrogen peroxide mixture is produced bymixing, for example, 2200 ml of the sulfuric acid (96 volume %) and 440ml of the hydrogen peroxide. A treatment time for the removal is set to15 minutes. The droplets 24 and the low-crystallinity gallium nitridelayer 22 have composition ratios Ga/N larger than one, and may beamorphous or polycrystalline. For example, the Ga/N of thelow-crystallinity gallium nitride layer 22 is sometimes larger than two,sometimes larger than three, and sometimes larger than four. As it isunderstood from the etching result shown in FIG. 7, the droplets 24 andthe low-crystallinity gallium nitride layer 22 are removed by a sulfuricacid-based etching solution. On the other hand, the GaN cap layer 20 ismonocrystalline, and the Ga/N of the GaN cap layer 20 is approximatelyone. As it is understood from the etching result shown in FIG. 8, theGaN cap layer 20 is hardly etched by the sulfuric acid-based etchingsolution.

As shown in FIG. 11C, a gate electrode 26 is formed on the exposed GaNcap layer 20. For example, a nickel (Ni) film and a gold (Au) film aredeposited in order on the substrate 10 to form the gate electrode 26having the laminated layer thereof. A source electrode 28 and a drainelectrode 30 are formed on the AlGaN electron supply layer 16, and thegate electrode 26 is located between the source electrode 28 and thedrain electrode 30. For example, a titanium (Ti) film and an aluminum(Al) film are deposited in order on the substrate 10 to form the sourceelectrode 28 and the drain electrode 30 each having the laminated layerthereof. A tantalum (Ta) film can be used instead of the Ti film. TheGaN cap layer 20 may be etched to form openings such that the sourceelectrode 28 and the drain electrode 30 are in direct contact with theelectron supply layer 16. But, the source electrode 28 and the drainelectrode 30 may be directly formed on the GaN cap layer 20. After theabove steps, a semiconductor device 104 is completed.

FIG. 12 is a top view schematically showing an optical microscope imageof an appearance of the article in the step to which the step shown inFIG. 11A has been applied. Referring to FIG. 12, a part is hatched toshow the droplet 24 on the low-crystallinity gallium nitride layer 22.This observation reveals that the surface roughness is in the surface ofthe low-crystallinity gallium nitride layer 22. This indicates that thecrystallinity of the low-crystallinity gallium nitride layer 22 isinferior.

FIG. 13 is a top view schematically showing an optical microscope imageof an appearance of the article in the step after the step shown in FIG.11B. Referring to FIG. 13, the droplet 24 has been removed together withthe low-crystallinity gallium nitride layer 22, and the trace, formed byremoving the droplet 24, in FIG. 9 is not observed. Sulfur (S) due totreatment by a drug solution containing S remains on the surface of theGaN cap layer 20 to terminate it.

FIG. 14 shows an example, of an X-ray photoelectron spectroscopy (XPS)measured result of the surface of a GaN layer on which sulfur remains.The abscissa indicates binding energy and the ordinate indicatesstrength. As shown in FIG. 14, a peak of S2p from Ga—S binding isobserved around 160 eV in the spectrum. Unless the treatment the drugsolution containing S is applied thereto, the spectrum does not containthe peak of S2p.

In the example 1, as shown in FIG. 10B, the nitride semiconductor layer18 is formed on the substrate 10. As shown in FIG. 10C, the GaN caplayer 20 (a first GaN layer of single crystal gallium nitride) is grownon the nitride semiconductor layer 18. As shown in FIG. 10D, theamorphous or polycrystalline low-crystallinity gallium nitride layer 22,which has a composition ratio of Ga larger than that of the GaN caplayer 20, of a gallium nitride compound (e.g., a second GaN layer ofgallium nitride with a composition ratio of gallium to nitrogen largerthan two) is grown on the surface of the GaN cap layer 20 to form anepitaxial substrate. As shown in FIG. 11A, after the substrate 10 isunloaded from the growth reactor, the low-crystallinity gallium nitridelayer 22 is removed from the epitaxial substrate to form anotherepitaxial substrate in which, as shown in FIG. 13, the droplet 24 hasbeen removed. The other epitaxial substrate thus formed does not includethe droplet 24 and the trace 70. By making semiconductor devices fromthe other epitaxial substrate, it is possible to improve the yield ofthe semiconductor devices.

As a growth condition for the low-crystallinity gallium nitride layer22, a substrate temperature is set to temperature equal to or lower thana decomposition temperature of a nitrogen material (e.g., NH₃). Forexample, when the low-crystallinity gallium nitride layer 22 is grownusing a growth reactor for MOCVD, the substrate temperature is set, tobe equal to or lower than 800 degrees Celsius, which makes a supplyamount of nitrogen smaller than a supply amount of Ga to allow theformation of the low-crystallinity gallium nitride layer 22 having Ga/Nlarger than one. For example, in the step to form the GaN cap layer 20,the substrate temperature is equal to or higher the decompositiontemperature of the nitrogen material. Subsequently, the substratetemperature is reduced to be equal to or lower than the decompositiontemperature of the nitrogen material, and the low-crystallinity galliumnitride layer 22 is formed to cover the GaN cap layer 20. In thisembodiment, a flow rate of the material gas (i.e., the total flow rateof the source gas and a carrier gas) is kept unchanged, and littlechange in gas flow occurs in the chamber 50. Droplets are hardly presentbetween the GaN cap layer 20 and the low-crystallinity GaN layer 22.

In another example of growth conditions for the low-crystallinitygallium nitride layer 22, the following condition can also be used aratio of the Ga material (the Ga material/the N material) is higher thanthat in the process for forming the GaN cap layer 20. For example, inthe process for forming, the low-crystallinity gallium nitride layer 22,a ratio of a TMG gas flow rate/a NH₃ gas flow rate is set smaller thanthat in the step for forming the GaN cap layer 20. This condition allowsthe thus-formed low-crystallinity gallium nitride layer 22 to have theratio, [Ga]/[N], of larger than one. In an example for growth in whichthe substrate temperature is reduced, continuous growth from the GaN caplayer 20 to the low-crystallinity gallium nitride layer 22 may beperformed. In an example for growth in which the material ratio ischanged, a ratio of the material gas is switched to form an abruptchange in composition at the boundary between the GaN cap layer 20 andthe low-crystallinity gallium nitride layer 22. When an overall flowrate of the gas that is introduced into the chamber 50 from theintroducing section 52 hardly changes, droplets are hardly formedbetween the GaN cap layer 20 and the low-crystallinity GaN layer 22. Inthe formation of the low-crystallinity gallium nitride layer 22, thesubstrate temperature and the Ga material ratio both may be changed fromthe substrate temperature and the Ga material ratio for forming the GaNcap layer 20, respectively.

In the example explained with reference to FIG. 11B, the mixed liquidincluding sulfuric acid and hydrogen peroxide was used to remove thelow-crystallinity gallium nitride layer 22. Another drug solution thatcan remove the low-crystallinity gallium nitride layer 22 and does notetch the GaN cap layer 20 can be also applied thereto. For example, adrug solution obtained by mixing purified water and the mixed liquid ofsulfuric acid and hydrogen peroxide may be applied thereto.

When the low-crystallinity gallium nitride layer 22 is removed using theetching liquid including sulfuric acid, sulfur atoms remain on thesurface of the GaN cap layer 20. For example, as shown in FIG. 14, apeak of the binding energy is observed around 160 eV in the spectrumobtained by measuring the surface of the GaN cap layer 20 by XPS method.

The growth reactor 100, shown in FIG. 1 in which the introducing section52 for introducing gas is provided above the wafer 60, is likely toeasily form droplets. Therefore, it is preferable to apply the method inthe example 1 thereto. Besides growth reactors. for MOCVD, the growthreactor 100 may be a hydride vapor phase epitaxy (HVPE) apparatus, aliquid phase epitaxy (LPE) apparatus, or a molecular beam epitaxy (MBE)apparatus.

The GaN cap layer 20 has a function of suppressing. oxidation of thenitride semiconductor layer 18. To suppress oxidation of the nitridesemiconductor layer 18, the film thickness of the GaN cap layer 20 ispreferably equal to or larger than 1 nm and more preferably equal to orlarger than 2 nm. When a trace with a depth equal to or smaller than 100nm as shown in FIG. 9 remains, the GaN cap layer 20 thicker in thicknessdoes not affect the nitride semiconductor layer 18. Accordingly, thefilm thickness of the GaN cap layer 20 is preferably equal to or smallerthan 100 nm. The film thickness of the GaN cap layer 20 is preferablyequal to or smaller than 10 nm and more preferably equal to or smallerthan 5 nm. This results in that the distance between the gate electrode26 and the electron supply layer 16 is made small to improve theperformance of the semiconductor device 104.

In order not to leave a trace with the depth of about 100 nm, the filmthickness of the low-crystallinity gallium nitride layer 22 ispreferably equal to or larger than 100 nm and more preferably equal toor larger than 200 nm. To suppress a crack due to film stress, the filmthickness of the low-crystallinity gallium nitride layer 22 ispreferably equal to or smaller than 2 μm and more preferably equal to orsmaller than 1 μm.

In the above embodiment, the HEMT is explained as an example of thesemiconductor device. However, the above embodiment of the semiconductordevice can be applied to other semiconductor devices. The nitridesemiconductor layer 18 can comprise, for example, GaN, InN, AlN, InGaN,AlGaN, InAlN, and InAlGaN. In the nitride semiconductor layer 18, theGaN cap layer 20, and the low-crystallinity gallium nitride layer, othercomponents may be included as long as the effects of the presentinvention are obtained.

The above embodiments can be provided to reduce the number of accretingobjects or particles, comprising gallium, which adhere onto asemiconductor layer.

Having described and illustrated the principle of the invention in apreferred embodiment thereof, it is appreciated by those having skill inthe art that the invention can be modified in arrangement and detailwithout departing from such principles. We therefore claim allmodifications and variations coming within the spirit and scope of thefollowing claims.

What is claimed is:
 1. A method for treating a surface of a semiconductor layer, the method comprising the steps of: growing a first layer on a substrate in a growth reactor, the first layer consisting of one of gallium nitride, aluminum gallium nitride and indium aluminium nitride; growing a second layer of gallium nitride on a surface of the first layer, the gallium nitride of the second GaN layer having a composition ratio of gallium to nitrogen larger than 2; taking the substrate out of the growth reactor after growing the second layer; and removing the second layer after taking the substrate out of the growth reactor.
 2. The method according to claim 1, wherein growing a second layer on a surface of the first layer includes a step of forming the second layer on the first layer using an MOCVD reactor at a substrate temperature equal to or lower than 800 degrees Celsius.
 3. The method according to claim 1, wherein, removing the second layer includes removing the second layer using mixed liquid including sulfur acid and hydrogen peroxide.
 4. The method according to claim 1, wherein the growth reactor includes an introducing port for introducing gas above the substrate.
 5. The method according to claim 1, wherein an accreting object are formed on the second layer after growing the second layer, and the accreting object is removed in the step of removing the second layer.
 6. The method according to claim 1, wherein the first layer and the second layer are continuously grown in the growth reactor.
 7. A semiconductor substrate comprising: a first layer consisting of one of gallium nitride, aluminum gallium nitride and indium aluminium nitride; and a second layer of gallium nitride having a composition ratio of gallium to nitrogen larger than 2, the second layer being provided on the first layer.
 8. The semiconductor substrate according to claim 7, further comprising particles containing gallium, the particles being provided on the second layer.
 9. The semiconductor substrate according to claim 7, wherein a film thickness of the second layer is equal to or larger than 100 nm.
 10. The semiconductor substrate according to claim 7, wherein the second layer is formed to be in contact with the first layer.
 11. A method for making an epitaxial substrate, comprising the steps of: growing a first layer on a substrate in a growth reactor, the first layer consisting of one of gallium nitride, aluminum gallium nitride and indium aluminium nitride; growing a second layer of gallium nitride compound on the first layer in the growth reactor, the first layer being in contact with the second layer, and the gallium nitride compound having a composition ratio of gallium to nitrogen larger than 2; taking the substrate out of the growth reactor after growing the second layer; and etching the second layer to expose the first layer after taking the substrate from the growth reactor.
 12. The method according to claim 11, wherein growing a second layer on a surface of the first layer includes a step of forming the second layer on the first layer using an MOCVD reactor at a substrate temperature equal to or lower than 800 degrees Celsius.
 13. The method according to claim 11, wherein, removing the second layer includes removing the second layer using mixed liquid including sulfuric acid and hydrogen peroxide.
 14. The method according to claim 11, wherein the growth reactor includes an introducing port for introducing gas above the substrate.
 15. The method according to claim 11, wherein an accreting object are formed on the second layer after growing the second layer, and the accreting object is etched in the step of etching the second layer. 